linux-kernel - Re: [PATCH 10/15] mm/hmm: do not differentiate between empty entry or missing directory v2

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Date:   Wed, 21 Mar 2018 10:48:27 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Evgeny Baskakov <ebaskakov@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>
Subject: Re: [PATCH 10/15] mm/hmm: do not differentiate between empty entry
 or missing directory v2

On Tue, Mar 20, 2018 at 10:24:34PM -0700, John Hubbard wrote:
> On 03/19/2018 07:00 PM, jglisse@...hat.com wrote:
> > From: Jérôme Glisse <jglisse@...hat.com>
> > 
> > There is no point in differentiating between a range for which there
> > is not even a directory (and thus entries) and empty entry (pte_none()
> > or pmd_none() returns true).
> > 
> > Simply drop the distinction ie remove HMM_PFN_EMPTY flag and merge now
> > duplicate hmm_vma_walk_hole() and hmm_vma_walk_clear() functions.
> > 
> > Changed since v1:
> >   - Improved comments
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> > Cc: Evgeny Baskakov <ebaskakov@...dia.com>
> > Cc: Ralph Campbell <rcampbell@...dia.com>
> > Cc: Mark Hairgrove <mhairgrove@...dia.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > ---
> >  include/linux/hmm.h |  8 +++-----
> >  mm/hmm.c            | 45 +++++++++++++++------------------------------
> >  2 files changed, 18 insertions(+), 35 deletions(-)
> > 
> > diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> > index 54d684fe3b90..cf283db22106 100644
> > --- a/include/linux/hmm.h
> > +++ b/include/linux/hmm.h
> > @@ -84,7 +84,6 @@ struct hmm;
> >   * HMM_PFN_VALID: pfn is valid. It has, at least, read permission.
> >   * HMM_PFN_WRITE: CPU page table has write permission set
> >   * HMM_PFN_ERROR: corresponding CPU page table entry points to poisoned memory
> > - * HMM_PFN_EMPTY: corresponding CPU page table entry is pte_none()
> >   * HMM_PFN_SPECIAL: corresponding CPU page table entry is special; i.e., the
> >   *      result of vm_insert_pfn() or vm_insert_page(). Therefore, it should not
> >   *      be mirrored by a device, because the entry will never have HMM_PFN_VALID
> > @@ -94,10 +93,9 @@ struct hmm;
> >  #define HMM_PFN_VALID (1 << 0)
> >  #define HMM_PFN_WRITE (1 << 1)
> >  #define HMM_PFN_ERROR (1 << 2)
> > -#define HMM_PFN_EMPTY (1 << 3)
> 
> Hi Jerome,
> 
> Nearly done with this one...see below for a bit more detail, but I think if we did this:
> 
>     #define HMM_PFN_EMPTY (0)
> 
> ...it would work out nicely.
> 
> > -#define HMM_PFN_SPECIAL (1 << 4)
> > -#define HMM_PFN_DEVICE_UNADDRESSABLE (1 << 5)
> > -#define HMM_PFN_SHIFT 6
> > +#define HMM_PFN_SPECIAL (1 << 3)
> > +#define HMM_PFN_DEVICE_UNADDRESSABLE (1 << 4)
> > +#define HMM_PFN_SHIFT 5
> >  
> 
> <snip>
> 
> > @@ -438,7 +423,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
> >  		pfns[i] = 0;
> >  
> >  		if (pte_none(pte)) {
> > -			pfns[i] = HMM_PFN_EMPTY;
> > +			pfns[i] = 0;
> 
> This works, but why not keep HMM_PFN_EMPTY, and just define it as zero?
> Symbols are better than raw numbers here.
> 

The last patch do that so i don't think it is worth respinning
just to make this intermediate state prettier.

Cheers,
Jérôme